Motor vehicle liftgates and deck lids act to close and seal the rear cargo area of a motor vehicle. Typically, these closures or closure structures are mounted in a frame located at the rear of the vehicle, usually on a horizontally extending axis provided by a hinge. The liftgate is thus positioned to rotate between a closed position adjacent to the frame and an open position, in which the cargo area of the motor vehicle is accessible. The liftgate or deck lid itself is often very heavy, and because of its mounting, it must be moved against gravity in order to reach the open position. Because of the liftgate's weight, it would be a great burden if a user was required to lift the liftgate into the open position and then manually hold it in place in order to access the vehicle's cargo area.
In order to make it easier to open liftgates and deck lids, most modern motor vehicles use gas or spring-loaded cylindrical struts to assist the user in opening and holding open liftgates and deck lids. The struts typically provide enough force to take over the opening of the liftgate after the liftgate has been manually opened to a partially opened position at which the spring force and moment arm provided by the struts are sufficient to overcome the weight of the liftgate, and to then hold the liftgate in an open position.
Usually, a motor vehicle liftgate-assist system consists of two struts. The two struts in a typical liftgate assembly are each pivotally mounted at opposite ends thereof, one end pivotally mounted on the liftgate and the other end pivotally mounted on the frame or body of the motor vehicle. Each strut's mounting point is fixed, and the strut thus possesses a fixed amount of mechanical advantage in facilitating the manual opening process. In addition, because the force provided by the struts is constant, the user must thrust downward on the liftgate and impart sufficient momentum to the liftgate to overcome the strut forces in order to close the liftgate.
Automated powered systems to open and close vehicle liftgates are known in the art. However, these systems typically use a power actuator to apply a force directly to the liftgate to enable opening and closing thereof. For example, U.S. Pat. No. 5,531,498 to Kowall discloses a typical liftgate-opening system in which the gas struts are actuated by a pair of cables which are, in turn, wound and unwound from a spool by an electric motor. Because this typical type of powered system acts as a direct replacement for the user-supplied force, it provides relatively little mechanical advantage from its mounted position, typically requires a significant amount of power to operate, and is usually large, acquiring a significant amount of space in the tailgate area of the vehicle, which is undesirable.
Control systems for the typical powered liftgate systems are also available. Such control systems usually include at least some form of obstacle detection, to enable the liftgate to stop opening or closing if an obstacle is encountered. These obstacle detection systems are usually based on feedback control of either the force applied by the liftgate or actuator motor or the speed at which the liftgate or motor is moving. One such control system for the type of cable-driven liftgate actuator described above is disclosed in U.K. Patent Application No. GB 2307758A. In general, the control system of this reference is designed to control the movement of the liftgate based on the measured liftgate force, using an adaptive algorithm to “learn” the liftgate system's force requirements. However, the movement of a liftgate is a complex, non-linear movement and existing control systems are usually adapted only for conventional “brute force” powered liftgate systems.
Other prior art power liftgate systems are more passive. For example, DE 198 10 315 A1 discloses an arrangement in which the angular position of a strut is changed in order to facilitate opening and closing of a deck lid. However, the structural configuration of the disclosed design is such that it permits a very limited range of closure movement and limited mechanical advantage in the different positions. In addition, among numerous other disadvantages, the device disclosed in DE 198 10 315 A1 does not provide a controlled system that enables dynamic control of the closure during movement thereof. This reference also does not contemplate use of the closure in manual mode, among other things.
DE 197 58 130 C2 proposes another system for automated closure of a deck lid. As with the '315 reference, the '130 reference does not contemplate or allow dynamic control over the deck lid, use of the deck lid in manual mode, and does not enable a power driven closing force to be applied to the lid. Moreover, both of the '130 and '315 references disclose very large structural arrangements, making packaging in a vehicle very difficult.
One particular challenge in power liftgate systems, especially those that are more passive, is dealing with situations in which the vehicle is parked or stopped on an incline. If the vehicle is parked or stopped on an incline, it may negate some or all of the mechanical advantage of the power liftgate system. Another challenge is designing a powered system such that the liftgate will open at a particular speed or within a particular time frame.